Re: [PATCH v2 net 6/7] net/sched: taprio: fix q->current_entry is NULL before its expiry

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Tue, Nov 07, 2023 at 06:20:22AM -0500, Faizal Rahim wrote:
> Fix the issue of prematurely setting q->current_entry to NULL in the
> setup_first_end_time() function when a new admin schedule arrives
> while the oper schedule is still running but hasn't transitioned yet.
> This premature setting causes problems because any reference to
> q->current_entry, such as in taprio_dequeue(), will result in NULL
> during this period, which is incorrect. q->current_entry should remain
> valid until the currently running entry expires.
> 
> To address this issue, only set q->current_entry to NULL when there is
> no oper schedule currently running.
> 
> Fixes: 5a781ccbd19e ("tc: Add support for configuring the taprio scheduler")

The "Fixes" tag represents the commit where the code started becoming a
problem, not when the code that you're changing was first introduced.

I find it hard to believe that the problem was in commit 5a781ccbd19e
("tc: Add support for configuring the taprio scheduler"), because we
didn't support admin -> oper dynamic schedules back then.

> Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
> ---
>  net/sched/sch_taprio.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index 01b114edec30..c60e9e7ac193 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -1375,7 +1375,8 @@ static void setup_first_end_time(struct taprio_sched *q,
>  			first->gate_close_time[tc] = ktime_add_ns(base, first->gate_duration[tc]);
>  	}
>  
> -	rcu_assign_pointer(q->current_entry, NULL);
> +	if (!hrtimer_active(&q->advance_timer))
> +		rcu_assign_pointer(q->current_entry, NULL);
>  }
>  
>  static void taprio_start_sched(struct Qdisc *sch,
> -- 
> 2.25.1
>